Tutorial: Step by Step Database Design in SQL Published on February 22, 2015 February 22, 2015 • 348 Likes • 43 Comments The main advantage of a GUID is that it’s unique; the chance of you hitting the same GUID twice is really unlikely. The design process should therefore always be viewed in this context. in single-language applications, always initialize the database with a proper locale. The Database Is Corrupt . Common Database Problems And Solutions. Let’s take a look at one example. Don’t forget about dictionaries and relations between tables. The short version is that I recommend you add an index wherever you expect it’ll be needed. This reduces data duplication and ensures referential integrity. What does this mean? Why does it take longer? The Author. And while it is importing, I will check time of execution of the previous query. You can avoid problems by declaring scalar variables with %TYPE qualifiers and record variables to hold query results with %ROWTYPE qualifiers. There is some debate in Common Database Problems with database design is improper normalization. Of course, if you keep your database backed up regularly, you're going to be all right. Luckily enough, in PostgreSQL it is sufficient to wrap the table name in double quotes, and it will work: That’s right, and it is worth digging deeper. When it comes to organizing data, I see the same mistakes in database design as I see in object design: Some developers like to turn everything into a … Allow creation of multiple indexes on a table, as well as unique indexes within a table. Data backup limiting length of text columns in the database is good in general, for security and performance reasons. We all get excited when a new project starts and, going into it, everything looks great. You have a performance problem? With a commitment to quality content for the design community. Now the database contains some exemplary data and we are ready to start the model inspection, including identifying potential problems that are invisible now but can arise in the future, when the system will be used by real customers. Remember that there are limits on the length of their names. You should similarly optimize data which are frequently updated. Most often we assume that sorting words in a language is as simple as sorting them letter by letter, according to the order of letters in the alphabet. We all get excited when a … It is relatively easy to start and difficult to master. My company (GenieDB) has developed a replicated database engine that provides "AP" semantics, then developed a "consistency buffer" that provides a consistent view of the database - as long as there are no server or network failures; then providing a degraded service, with some fraction of the records in the database becoming "eventually consistent" while the rest remain "immediately … You can also add them after the database is in production if you see that adding index in a certain spot will improve performance. sometimes you may want to use language specific to data being browsed. If we do it this way, we could store that calculated value and use it later without having to recalculate it. However, as you probably remember, “order” is a reserved word in SQL! If the system you’re building is another iteration of an existing project, you can estimate the expected size of the data in your system by looking at the data volume in the old system. (Most likely, it will be “id”. Lack of Communication Between DBAs and Developers Writing documentation happens just before the project is closed — and just after we’re mentally done with that data model! What you decide to use is closely related to the problem you’ll solve, but it’s important to include the client’s preferences and their current IT infrastructure. The bottom line is – don’t use keywords as object names. Remember that you name not only tables and their columns, but also indexes, constraints and foreign keys. It should always be a timestamp type. Here are the ten worst mistakes. In this article, I’ve listed 24 different database design mistakes that you should try to avoid. Some are very common regardless of the business, e.g. To do so, I will use a very long word list, and turn it into SQL using a simple Perl command. Remember to keep the terminology similar to whatever the client currently uses. In Oracle, the varchar column type is limited to 4000 bytes. If you have to use it, only use it when you’re 100% sure that it is really needed. The topic of giving good names to tables and other elements of a database – and by good names I mean not only “not conflicting with SQL keywords”, but also possibly self-explanatory and easy to remember – is often heavily underestimated. Here is the model with book_comment type changed to text: There is a saying that “greatness is achieved, not given”. Adam Evanovich lives in Iowa in the United States and frequently works on contract in various industries. So if you define a column with type varchar(3000 char), then it means that you can store 3000 characters in that column, but only if it does not use more than 4000 bytes on disk. A similar approach should be taken when logging events in a multi-time zone system. Then we will have a good chance to restore it and avert the loss. Which tables will be the central tables in your model? Notice that: Now imagine the mess would we create if our model contained hundreds of tables. This could seem really great. But that’s not the case. What rules will apply when naming tables and other objects? To embrace the digitization efforts, AHIMA has adopted an initiati… However, the developer can easily make mistakes to cause a form to behave incorrectly or poorly. They tend to think normalization is the only way of designing. Founded by Vitaly Friedman and Sven Lennartz. It could contain properties like “customer value”, “contact details”, “additional info” etc. Ignoring data safety may lead to unexpected data loss or high costs of recovery of lost data. stored procedures can be satisfied with non-store procedure solutions. Here we will focus on the model design, of course. The art of designing a good database is like swimming. On the other hand, some tables in your model will be quite specific. If you find them, probably adding some indexes will speed things up a lot. Please tell us in the comments below. When you start the database design process, you’ll probably understand most of the main requirements. I’ve split the list of errors into two main groups: those that are non-technical in nature and those that are strictly technical. If you do this, have a really good reason. Once again, we’ll lose a little hard drive space, but we’ll avoid recalculating data or connecting to the reporting database (if we have one). And that includes almost everything, from writing simple SELECT queries to getting all customer-related values to inserting, updating, or deleting values. And if it is so – there is no error here. How do we check that? You can read more about naming conventions in these two articles: Normalization is an essential part of database design. But if they format it using bold font, like this: then it will take 24 characters to store while the user will see only 17 in the GUI. In case of handling time zones the database must cooperate with application code. It happens. What name will we use for the ID columns? It will cost some time, but you will deliver a better product and sleep much better. Reporting data should be only stored in this manner if we need to use it often. First, we’ll add a dictionary with a list of all the possible properties we could assign to a customer. Poor Naming Standards. Now we're entering the territory of a bigger problem. This issue also arises when we use UNIQUE real-world values (e.g. The basic info is retrieved very often. A quick fix could save some time now and would probably work fine for a while, but it can turn into a real nightmare later. Whenever possible, covered topics will be illustrated by models generated using Vertabelo and practical examples. You’ve probably made some of these mistakes when you were starting your database design career. But you should also practice as much as possible, because the sad truth is that we learn most… by making errors. Use business, domain-specific knowledge your customer has to estimate the expected volume of the data you will process in your database. Database technology has been a familiar tool in the operations of most HIM departments and a very basic understanding of this technology has usually been adequate enough to allow HIM professionals to work effectively with vendors or information services staff. sometimes you need to optimize the query, and not the model; not every performance problem can be solved by creating an index; there are other possible ways of solving performance problems: tuning your database parameters and buffer sizes. in multi-language applications, initialize the database with some default locale, and for every place when sorting is available, decide which collation should be used in SQL queries: probably you should use collation specific to the language of the current user. Timestamp in SQL Server is something completely different than timestamp in PostgreSQL. There are quite a lot of catches there. With separation, we keep the frequently used table small, but we still keep the data. They add value to your code and they relate the technology to the real-world problem you need to solve. We have populated the database with some manually created test data. our. This article covers designing databases in general, but with emphasis on web applications, so some examples may be web application-specific. So – language of content can affect ordering of records, and ignoring the language can lead to unexpected results when sorting data. If you mean Christmas Eve midnight in your own time zone, you must say “December 24, 23.59 UTC” (or whatever your time zone is). browse and search books by book title, description and author information. Use them as alternate keys instead. Keeping versions and logging events makes your database more complex. The more you sell the more rows there will be in the purchase table. Do it as it should have been done, no matter what the current situation. Anyone producing reports will have to use the same resources as users working on operational tasks; reporting queries are usually more complex and can affect performance. If you wrap something in double quotes in SQL, it becomes a delimited identifier and most databases will interpret it in a case-sensitive way. These calculations could use many tables and consume a lot of resources. The system should allow customers to perform the following activity: So the initial database model could look like this: To test the model, we will generate SQL for the model using Vertabelo and create a new database in PostgreSQL RDBMS. second, sorting letter-by-letter is sometimes wrong when accents come into play. This is the part of the project where we can make crucial mistakes. Ever. Everyone agrees that great database performance starts with great database design. Time of events should always be logged in a standardized way, in one selected time zone, for example UTC, so that you could be able to order events from oldest to newest with no doubt. For example, special offers’ expiry times (the most important feature in any store) must be understood by all users in the same way. The design mistakes listed in this article may seem small and insignificant at the start. It is relatively easy to start and difficult to master. But non-technical skills? We’ll start with the simple and obvious – naming standards. Non-Technical Database Design Errors #1 Poor Planning . Handling time zones properly requires cooperation between database and code of the application. Do you think we missed something important? Luckily, it covers most characters used in all the world. Databases have different data types to store date and time. The query plan now is quite different: “Index Scan” means that instead of browsing the table, row by row, the database will browse the index we’ve just created. Luckily, we can help it by telling PostgreSQL to sort this table by send_ts, and save the results. Skipping is only an option if 1) you have a really small project; 2) the tasks and goals are clear, and 3) you’re in a real hurry. scheduling bulk deletes at night, to avoid unnecessary table locks. That solution doesn’t need to be a complete application – maybe a short document or even a few sentences in the language of the client’s business. adjusting your database transaction isolation level. The two previous blogs addressed primary key and foreign key errors as well as confusion with many-to-many relationships. As an example learners could discuss the impact of errors such as the accidental deletion of a field in a query or report the renaming of a field changing data types etc. If the field will be plain-text in the GUI (customers can enter only unformatted comments) then it simply means that the field can store up to 1000 characters of text. Customers come from all over the world and use different time zones. You might misunderstand some requirements, the client might add some new functionalities, you’ll see something that could be done differently, the process might change, etc. And here PostgreSQL tells us it is going to do a “Seq Scan on book_comment” which means that it will check all records of book_comment table, one by one, to sort them by value of send_ts column. Current purchases are retrieved all the time: their status is updated, the customers often check info on their order. For most of us, documentation comes at the end of the project. Explain everything needing additional explaining, and basically write down everything you think will be useful one day. Before you sit down to draw a data model, you need to be sure that: During the planning phase, you should get answers to these questions: Only when you have all these answers are you ready to share an initial solution to the problem. ), there comes the second question – who did it? The design process should therefore always be viewed in this context. D1 is an opportunity for users to identify and explain a range of common errors in database design and construction explaining how these types of errors can be avoided. Both these groups are an important part of database design. An operational database is not meant to store reporting data, and mixing these two is generally a bad practice. In a multi-time zone system date column type efficiently does not exist. The “customer_attribute” table contains a list of all attributes, with values, for each customer. Imagine a system where parts of user info are frequently updated by an external system (for example, the external system computes bonus points of a kind). If you want to learn to design databases, you should for sure have some theoretic background, like knowledge about database normal forms and transaction isolation levels. Database designer and developer, financial analyst. Because you didn’t properly document, you don’t know where to start. The reason I am writing a full blown article is, when developers design a database they tend to follow the three normal forms like a silver bullet. Now let’s look at errors with many-to-many relationships. You never know for sure how long a project will last and if you’ll have more than one person working on the data model. This is when it’s wise to decide how you will name objects in your model, in the database, and in the general application. our. Talk with developers and clients and don’t be afraid to ask vital business questions. The art of designing a good database is like swimming. In such cases, it would be wise to perform these calculations during off hours (thus avoiding performance issues during working hours). As we can see in book_comment table, the comment column’s type is character varying (1000). Now, if I wanted to complicate things even more, I can create another table, this time named ORDER (in uppercase), and PostgreSQL will not detect a naming conflict: The magic behind that is that if an identifier is not wrapped in double quotes, it is called “ordinary identifier” and automatically converted to upper case before use – it is required by SQL 92 standard. To start and difficult to master last discussion these comments, we could assign a! We need to wrap everything up that it is importing, I use. Send_Ts, and it exceeds the scope of this article may seem small and insignificant at the.. Following query: how fast does this query run our operational database not. 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